Conventionally, a coating composition, an adhesive, an adhesive mass, an ink, a filler and a molding material contain resin solutions containing organic solvents. These resin solutions dissipate large amounts of organic solvents in the steps of coating and charging and in the steps of curing and drying. With increasing concerns about global environments and working environments, restrictions have come to be imposed on the use of the above resin solutions. one of methods for overcoming the above problem uses an aqueous solution of a resin, a powder or a hot-melt material. However, an aqueous solution of a resin contains an organic solvent to some extent for improving coating properties, and it can hardly be said that the aqueous solution of a resin is free of an odor in a working environment. Further, not only it is required to incinerate a discharged organic solvent, but also the disposal of its effluent requires an investment. In a coating or charging plant having large-scale equipment for the treatment of discharged gas, the release of an organic solvent to atmosphere can be prevented, while a small-scale plant having no such equipment has a problem that an effluent containing an organic solvent cannot be treated. The coating or charging of a powder or a holt melt requires equipment and a method which are quite different from conventional ones, and it is therefore required to introduce new equipment. For overcoming the above problems, attempts are being made to form a high-solid resin solution and to improve an aqueous solution of a resin, and it is considered that the amount of a resin solution remarkably tends to decrease owing to the above efforts. As an essential solution, however, it is strongly desired to develop a solventless liquid resin which is free from problems concerning pollution, hygienic safety, ignition, explosion, etc., which can be widely applied and which can be easily applied. Further, the solventless liquid resin is required to form a cured film or a shaped article by means of a conventional drying appratus.
Further, concerning a conventional radiation-curable resin composition, its viscosity is controlled by incorporating a large amount of a component having a low molecular weights However, the incorporation of a component having a low molecular weight is not desirable in view of a working environment since it causes problems of odor, etc. There are further another problems that a volume shrinkage takes place at a curing time and a cured film becomes fragile. For improving the curing-caused shrinkage ratio, attempts have been made to use a monomer component having a relatively high molecular weight or to add a component having a high molecular weight. However, since the latter component is in the state a solid, the amount of the component that can be added is limited in order to keep the viscosity of the composition in a proper range. Further, there are problems caused by a large content of a compound having a low molecular weight such as a problem on an odor caused by a residual monomer, and these problems limit the radiation-curable resin composition in use.
JP-A-57-171 discloses a technique concerning a solventless resin composition. This technique uses a liquid resin containing an acrylic monomer, while the technique is desired to be further improved since an obtained resin is an oligomer. Further, in view of physical properties, it is known that it is difficult to control the physical properties of a cured film obtained from a coating composition containing a resin which is in an oligomer region (MUROI Soichi, "Lectures in Society of Adhesion and coating Studies in 1992", Summary Prints of Lectures, page 4, 1993), and it is desired to increase a molecular weight while maintaining a low viscosity.
In reactions for curing resins with various kinds of radiation as a trigger, a radical-based crosslinking reaction and a cation-based crosslinking reaction are well known. A composition using a radical-based crosslinking reaction causes an extreme curing shrinkage. On the other hand, a cation-curable composition containing an alicyclic epoxy compound is improved in curing shrinkage to some extent. However, in particular, when an alicyclic epoxy resin having a low molecular weight and having a low viscosity and a high dilution effect is used, a volume change occurs at a curing time. JP-A-2-289611 discloses an invention of an acrylic resin having an alicyclic epoxy group. It can be said that the above invention further moderates a volume change at a curing time. In this technique, however, the acrylic resin contains a large amount of an organic solvent since it is synthesized by a solution method, and it is required to remove the solvent by some method in order to obtain a solventless resin composition. Further, since a cationic polymerization initiator is used as an essential component, it can be said that there are problems on the safety and hygiene which remain to solve.
Not only for the reaction of an alicyclic epoxy group, but also for the curing reaction with ultraviolet light, it is required to add a large amount of an initiator. The initiator and a sensitizer migrate out of a cured film, and this migration phenomenon has been a problem in view of safety and hygiene. When a high-energy radiation such as an electron beam or y ray is used as a trigger, a radical curable composition requires no catalyst. However, it is pointed out by J. V. Crivello et al that the cationic reaction of an alicyclic epoxy resin essentially requires an initiator, a particularly harmful antimony-containing catalyst, and the cationic reaction of an alicyclic epoxy resin deviates from the advantage of an electron-beam-curable composition that no initiator is required.
The present inventors have made diligent studies of a correlation between the structure and the viscosity of various resin compositions, and as a result, have found a solventless, radiation-curable liquid resin composition which contains a large amount of a component having a high molecular weight but has a viscosity sufficient for film-formability by a conventional film-forming method, and which can be cured at a high rate by a conventional curing method using radiation as a trigger. Further, by introducing an alicyclic epoxy group for a side chain component of a liquid resin having a high molecular weight, there can be obtained a radiation-curable liquid resin composition containing an alicyclic-epoxy-containing resin having a high molecular weight and containing neither an alicyclic epoxy compound having a low molecular weight nor a solvent.
It is an object of the present invention to provide a solventless curable liquid resin composition containing a liquid polymer having a high molecular weight and containing no solvent. The above curable liquid resin composition serves to decrease the amount ratio of a compound having a low molecular weight, which compound causes a problem with regard to safety and physical properties, and it serves to improve working environments. Further, it is another object of the present invention to provide a curable liquid resin composition capable of forming a film by a conventional coating method using a roll coater or a knife coater or a conventional printing method such as an offset printing method, a gravure printing method, a letterpress printing method or a screen printing method. It is further another object of the present invention to provide a radiation-curable liquid resin composition which can be cured by the irradiation with a conventional radiation such as ultraviolet light, electron beam or .gamma. ray, or which can be cured in the absence of a catalyst and an initiator by the irradiation with electron beam or .gamma. ray in particular.